What Files Do CNC Machines Use?

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  Key takeaways
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  From CAD file to spindle: why the format matters
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  STEP: the default for machined metal parts
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  STL, OBJ and 3MF: when a triangle mesh is enough
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  IGES, DXF and DWG: older translators and flat profiles
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  G-code and M-code: what the controller actually runs
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  Drawings, PDFs and native CAD: the supporting layer
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  How to prepare a file package that quotes in 12 hours
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  Which CNC file format to send for which part

Every machined part travels through three stages. Your CAD system produces a model. A CAM programmer converts that model into toolpaths. The machine controller executes those toolpaths as G-code. The question of what files do CNC machines use has two answers, because the controller only ever sees the third stage.

That gap is where most RFQ problems start. A file can look perfect on your screen and still arrive with gaps, reversed normals, or a single merged body where you intended three separate parts. The CAM programmer then has to rebuild geometry before any cutting starts.

Format choice sets how much rebuilding is needed. Boundary-representation formats such as STEP and IGES carry true analytic surfaces: a Ø20 mm bore stays a Ø20 mm cylinder. Mesh formats such as STL and OBJ approximate that same bore with flat triangles, and the approximation is baked in.

There is no universally correct format. There is a correct format for each part type, tolerance band, and downstream process. The sections below cover each one.

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  CAD modelYour design intent: solids, surfaces, features, assemblies.
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  CAM toolpathStock model, cutters, feeds, speeds, workholding, toolpath strategy.
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  G-codeThe only file the machine controller actually reads and runs.

STEP (ISO 10303) is the format we ask for first on any 3D job. It stores true B-rep geometry: planes, cylinders, cones, torus surfaces and NURBS, plus assembly structure and material properties. When your model has a Ø12 H7 bore, the STEP file describes a Ø12 cylinder, not a stack of triangles.

That precision is what CAM software needs. Toolpath algorithms detect cylindrical faces and generate helical or circular interpolation directly. Mesh geometry forces the CAM system to fit arcs back onto triangles, and the fit introduces small deviations that show up in the finished part.

Send STEP AP214 or AP242. Both carry color and assembly data, which helps when a part is one of forty in a fixture assembly. AP203 works too, but drops some metadata.

One caution: STEP files do not carry tolerances, datums, thread callouts or surface finish. Those live on the 2D drawing. Send both.

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  Best forMilled and turned 3D parts, complex pockets, blended surfaces, assemblies.
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  Watch forExport the part, not the whole top-level assembly, unless you want every component quoted.
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  Pair withA PDF drawing carrying tolerances, threads and finish callouts.

STL stores a surface as a cloud of triangles. Each triangle is defined by three vertices and a normal vector. There is no unit, no material, no color, and no way to say which triangles form a face you care about. It is the lowest-common-denominator format.

That is fine when the geometry is organic or the tolerance is loose. A bracket with a ±0.5 mm profile tolerance on a curved outer edge will machine perfectly well from a good STL. Meshes are also the native output of most 3D scanners and lattice design tools.

It falls apart when precision matters. A Ø25 mm bore tessellated at 0.05 mm chord deviation is still a polygon, and a reamer will not produce a round hole from a polygon. Same for flatness on a sealing face. If the feature has a tolerance tighter than roughly ±0.05 mm, do not send a mesh.

OBJ and 3MF improve on STL by adding units, color and multiple objects. 3MF is the better choice for additive work. For subtractive machining, the geometry underneath is still a mesh, so the same limitation applies.

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  Chord deviationSet it to 0.01–0.02 mm before export, or the facets become visible.
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  Watertight checkNon-manifold edges stall CAM software. Run a repair pass first.
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  Units missingSTL has no unit field. State millimetres in the RFQ email.

IGES (Initial Graphics Exchange Specification) predates STEP and still appears in aerospace and tooling supply chains. It handles 2D and 3D wireframe, surfaces and solids, but it is a looser standard. Trimming boundaries can fail and surfaces can arrive as thousands of small patches that need stitching.

Use IGES when your customer's legacy CAD system only exports IGES. Otherwise convert to STEP. If you must send IGES, send surfaces rather than wireframe, and check that the model came through as one closed body rather than a patchwork.

DXF is a 2D format. It carries lines, arcs, polylines, splines, layers and dimensions. It does not carry a 3D solid. For flat parts — sheet metal blanks, gaskets, plates, brackets cut from 1–20 mm stock — a clean DXF is the ideal input for laser, waterjet and plasma cutting, and it is also useful to a mill programmer for profiling a 2D contour.

DWG is AutoCAD's native format and holds 2D and 3D data. Many shops can open it directly. If you are unsure which one the shop prefers, send both DXF and DWG, plus a PDF for reference.

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  DXF layersDelete construction and dimension layers before export. Keep one layer for the cut profile.
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  Closed contoursAny open polyline becomes an unclosed cut path. Close every profile.
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  IGES surfacesAsk for a single closed shell; loose patches add programming time.

G-code is the instruction set the machine executes. G00 moves at rapid feed. G01 cuts in a straight line at a programmed feed rate. G02 and G03 cut arcs. G54 selects a work offset. M03 and M05 start and stop the spindle. M08 turns coolant on.

The CAM programmer generates this from your STEP file plus a stock model and a tool library. Feeds and speeds come from material, cutter geometry and rigidity. A 6 mm three-flute carbide end mill in 6061-T6 might run at 12,000 rpm and 2,500 mm/min, while the same cutter in 17-4PH stainless drops to a fraction of that.

You rarely need to send G-code. It is machine-specific: a program written for one controller and fixture setup usually will not run on another. The exceptions are legacy parts with no surviving CAD model, where a proven program is the only remaining record.

What you can send that helps: a CAM file, a setup sheet, or the original toolpath project. It tells us how the part was previously held and cut, which shortens programming time on a repeat order.

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  G-code is disposableIt is regenerated whenever stock, tooling or fixture changes.
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  Send the model, not the programUnless the CAD model is lost and the program is all you have.
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  Setup sheets are goldThey document workholding on parts that are hard to fixture.

Geometry answers where to cut. The drawing answers how close, how smooth and how to inspect. A PDF is the practical way to send it because it opens everywhere and cannot be silently edited. Native CAD files (SLDPRT, IPT, CATPart, X_T) are usable if the shop runs the same software, but version mismatches are common and files can carry reference geometry you did not intend to release.

A useful drawing carries a title block with part number and revision, a datum scheme, tolerance callouts, thread specifications, surface finish by area, and a material and finish note. It also marks critical dimensions. Everything else can run to the general tolerance block.

If the part has no drawing, say so and give us the tolerance band in the email. For most non-critical brackets and covers, a general tolerance of ±0.1 mm on machined dimensions and ±0.2 mm on non-mating features is enough. GreatLight holds ±0.005 mm where a drawing requires it, and inspects 100% of parts before shipment.

One more file worth mentioning: the neutral Parasolid X_T. It transfers solids cleanly between CAD systems and is a good fallback when a STEP export is failing.

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  PDF over native CADNo version conflicts, no accidental release of internal geometry.
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  Call out finishRa 0.8–1.6 μm covers most mating faces; Ra 0.2–0.8 μm for seals and bearings.
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  Mark critical dimsInspection time follows the number of toleranced features, not the part size.